1. Field of the Invention
The present invention relates to a vehicle using a heat engine such as an internal combustion engine and an electric motor in combination as a power source, i.e., a so-called hybrid vehicle.
2. Description of the Related Art
Hybrid vehicles using both an internal combustion engine and an electric motor may be roughly classified into two types. One type is referred to as the "SHV system" in which the internal combustion engine is connected to a power generator and performs only generation of electric power and charging of the battery and the electric motor receives the supply of the electric power from the power generator and the battery to drive the wheels (which can serve also as the power generator for the brakes). The other type is referred to as the "PHV system" in which both of the internal combustion engine and the electric motor (which can serve also as the power generator for the brakes) are mechanically connected to the wheels and can drive the wheels in parallel. The SHV system has a problem that the fuel consumption is degraded due to the conversion loss when converting all of the power of the internal combustion engine to electric power and then converting the same to power again. The PHV system has a problem that the exhaust emission deteriorates since the speed and output torque of the engine fluctuates while the vehicle is being driven.
As a countermeasure with respect to these problems, as disclosed in for example Japanese Unexamined Patent Publication No. 7-15805, there has been proposed a system in which the internal combustion engine and the electric motor are connected with the drive shaft by a sun-and-planet gear mechanism or a differential gear mechanism, the wheels are driven by the internal combustion engine so as to reduce the conversion loss, and, at the same time, the electric motor is controlled so that the speed of the internal combustion engine becomes constant, and thereby the deterioration of the exhaust emissions is prevented. However, the sun-and-planet gear mechanism and the differential gear mechanism function to divide the torque to be transmitted by a constant ratio, therefore if the driving torque of the wheels fluctuates in accordance with the state of driving, the required torque which should be generated by the internal combustion engine also changes. Thus, this is not sufficient as a means for preventing the deterioration of the exhaust emissions.
Further, as another system, as disclosed in Japanese Unexamined Patent Publication No. 6-233411, there exists a system which deals with the problem that when an internal combustion engine is connected to the wheels and operated under the conditions of a constant torque, the constant torque the engine exhibits will either be excessive or insufficient with respect to the driving torque of the wheels, which fluctuates in accordance with the state of driving. Therefore, that amount is adjusted by the control of the electric motor. In this case, however, even if the torque of the engine can be made constant, the speed of the engine will be changed in accordance with the state of driving of the vehicle, therefore the exhaust emissions still cannot be sufficiently improved.
Contrary to this, a technology separately providing an electric motor at the driving shaft to maintain the torque generated by the engine at a predetermined value is disclosed in Japanese Unexamined Patent Publication No. 50-30223. However, the technology disclosed in this publication uses a plurality of sun-and-planet gear mechanisms, and therefore that structure becomes complex.
Further, Japanese Unexamined Patent Publication No. 05-008639 discloses a hybrid automobile having a simple structure as a whole in which an electric motor and a continuously variable transmission, referred to as a CVT, are mounted in the vehicle in place of the sun-and-planet gear mechanism. In this publication, it is described that a low fuel consumption and a low emission can be realized by stopping the engine at the time of deceleration, but no method is disclosed for freely controlling the output of the engine. Further, in this case, there is a problem in that the emission is considerably increased by the frequent stopping of the engine or the vibration and noise are increased due to the torque fluctuation generated at the stopping and starting of the engine.